Irrigation Channels

Irrigation Channels

Module-IIPart-II

Syllabus• Irrigation channels• Alignment- canal capacity- losses- FSL of

canal- design of canal in alluvial soil andnon alluvial soils- Kennedy’s silt theory-Lacey’s regime theory- balancing depth- use ofGarrets diagrams and Lacey’s Regime diagrams-lining of irrigation channels- design of linedcanal- drainage behind lining. Water logging:Causes, Measures: surface and sub-surfacedrains, land reclamation

Alluvial and Non-Alluvial Canal • The soil which is formed by transportation and

deposition of silt through the agency of water, over acourse of time, is called the alluvial soil.

• The canals when excavated through such soils are calledalluvial canals. Canal irrigation (direct irrigation using aweir or a barrage) is generally preferred in such areas,as compared to the storage irrigation (i.e. by using a dam).

• The soil which is formed by the disintegration of rockformation is known as non-alluvial soil. It has an uneventopography, and hard foundations are generally available.The rivers, passing through such areas, have notendency to shift their courses, and they do not posemuch problems for designing irrigation structures on them.Canals, passing through such areas are called non-alluvialCanals.

Alluvial and Non-Alluvial Canal

Definition of Important Terms• Gross Command Area (GCA)

The whole area enclosed between an imaginaryboundary line which can be included in an irrigationproject for supplying water to agricultural land by thenet work of canals is known as GCA. It includes boththe culturable and unculturable areas.• Uncultivable Area

The area where the agriculture can not be doneand crops cannot be grown – marshy lands, barrenlands, ponds, forest, villages etc. are considered asuncultivable area.• Cultivable AreaThe area where agriculture can be done satisfactorily

Definition of Important Terms• Cultivable Command Area (CCA)The total area within an irrigation project where thecultivation can be done and crops can be grown

• Intensity of IrrigationRatio of cultivated land for a particular crop tothe total culturable command areaIntensity of irrigation, II = Land Cultivated

CCA

Definition of Important Terms• Time FactorThe ratio of the number of days the canal hasactually been kept open to the number of days thecanal was designed to remain open during the baseperiod is known as time factor.

Definition of Important Terms• Capacity FactorGenerally, a canal is designed for a maximum dischargecapacity. But, actually it is not required that the canal runs tothat maximum capacity all the time of the base period. So, the ratio ofthe average discharge to the maximum discharge (designeddischarge) is known as capacity factor.

For example, a canal was designed for the maximum dischargeof 50 cumec, but the average discharge is 40 cumec.

Capacity factor = 40/50 = 0.8

Channel Losses• During the Passage of water from the main Canal to the

outlet at the head of the water course, water may be losteither by evaporation from the surface or by seepage throughthe peripheries of the channels, So in determining thedesigned channel capacity, a provision for these water lossesmust be made.

(i) Evaporation• The water lost by evaporation is generally very small a

compared to the water lost by seepage in certain channels.Evaporation losses are generally of the order of 2 to 3 % ofthe total losses. They depend upon all those factors on whichthe evaporation depends, such as temperature, wind velocity,humidity, etc. In summer season, these losses may be morebut seldom exceed 7 %.

Reducing Evaporation Through Innovation

Tapping solar power, avoiding Evaporation Losses

Channel LossesSeepage: There may be two different condition of seepage, i.e. (i)Percolation, (ii) Absorption.(i) Percolation• In Percolation, there exists a zone of continuous saturation from

the canal to the water-table and a direct flow is established. Almostall the water lost from the canal, joins the ground water reservoir.

• The Losses of water depends upon the difference of top watersurface level of the channel of the water table.

(ii) Absorption• In Absorption, a small saturation soil zone exists around the canal

section and is surrounded by zone of decreasing saturation. Acertain zone just above the water table is saturated by capillarity.Thus, there exists an unsaturated soil zone between the twosaturated zones.

• In this case, the rate of loss is independent of seepage head (H) butdepends only on the water head h plus the capillary head hc.

Seepage Losses

Canal lining to prevent seepage losses

Cross-Section of an Irrigation Canal

Side Slopes• The side slopes should be such that they are stable,

depending upon the type of the soil. A comparativelysteeper slope can be provided in cutting rather than infilling, as the soil in the former case shall be more stable.

In cutting ------- 1H: 1V to 1.5 H: 1V In filling ------ 1.5 H: 1V to 2H: 1V

Berms• Berm is the horizontal distance left at ground level between the

toe of the bank and the top edge of cutting.

• The berm is provided in such a way that the bed line and thebank line remain parallel. If s1: 1 is the slope in cutting and s2:1 infilling, then the initial berm width = (s2 – s1) d1.

Purposes of Berms• They help the channel to attain regime conditions.• They give additional strength to the banks and

provide protection against erosion and breaches.• They protect the banks from erosion due to wave

action.• They provide a scope for future widening of the

canal.

Free Board• The margin between FSL and bank level is known as

freeboard. The amount of freeboard depends upon thesize of the channel. The generally provided values offreeboard are given in the table below:

Banks• The primary purpose of banks in to retain water. This can be

used as means of communication and as inspection paths.They should be wide enough, so that a minimum coverof 0.50 m is available above the saturation line.

Service Roads• Service roads are provided on canals for

inspection purposes, and may simultaneously serveas the means of communication in remote areas.They are provided 0.4 m to 1.0 m above FSL,depending upon the size of the channel.

Spoil Banks • When the earthwork in excavation exceeds earthworks

in filling, even after providing maximum width of bankembankments, the extra earth has to be disposed ofeconomically. To dispose of this earth by mechanicaltransport, etc. may become very costly, and an economicalmode of its disposal may be found in the form of collectingthis soil on the edge of the bank embankment itself.

Borrow Pits • When earthwork in filling exceeds the earthwork in

excavation, the earth has to be brought from somewhere. Thepits, which are dug for bringing earth, are known as BorrowPits.

Problem• Calculate the balancing depth for a channel section

having a bed width equal to 18 m and side slopes of 1:1 incutting and 2:1 in filling. The bank embankments are kept3.0 m higher than the ground level (berm level) and crestwidth of banks is kept as 2.0 m

Problem

Problem• Find the Balancing depth for a Canal Section having the

following data.• Base width of canal= 10 m• Side Slope in Cutting= 1:1• Side slope in Banking= 2:1• Top width of bank= 3 m

SolutionArea of Banking= 2 x 15 + 3 x 3= 54 sq. .m ………..(1)

2 Let d be the balance depth of cutting.Area of cutting= 10 + 10 + 2d x d = ( 10 + d) d …………(2)

2Equating the area of banking and cutting,(10 + d) x d= 54

D2 + 10d – 54= 0 d= -10 ±√100 + 216 = -10 ± 17.8

2 2d= -10 + 17.8 = 3.89 m (Neglecting –ve sign)

2

Alignment of Canal• Water-shed Canal (Ridge Canal)• Contour Canal • Side-slope Canal

Water-shed Canal (Ridge Canal)

Contour Canal

Side-Slope Canal

Distribution System for Canal Irrigation

Canal Design Types

Design Parameters• The design considerations naturally vary

according to the type of soil.• Velocity of flow in the canal should be critical.• Design of canals which are known as

‘Kennedy’s theory’ and ‘Lacey’s theory’ arebased on the characteristics of sedimentload (i.e. silt) in canal water.

Important Terms Related to Canal Design

• Alluvial soil • Non-alluvial soil • Silt factor • Co-efficient of Rugosity • Mean velocity • Critical velocity • Critical velocity ratio (c.v.r), m • Regime channel • Hydraulic mean depth • Full supply discharge • Economical section

Alluvial Soil• The soil which is formed by the continuous

deposition of silt is known as alluvial soil. Theriver carries heavy charge of silt in rainy season.When the river overflows its banks duringthe flood, the silt particles get deposited onthe adjoining areas.

• This deposition of silt continues year after year.This type of soil is found in deltaic region of ariver. This soil is permeable and soft and veryfertile. The river passing through this type of soilhas a tendency to change its course.

Alluvial Soil

Non-Alluvial Soil• The soil which is formed by the disintegration

of rock formations is known as non-alluvial soil.It is found in the mountainous region of a river.The soil is hard and impermeable in nature. Thisis not fertile. The river passing through this type ofsoil has no tendency to change its course.

Silt Factor• During the investigations works in various

canals in alluvial soil, Gerald Lacey establishedthe effect of silt on the determination ofdischarge and the canal section. So, Laceyintroduced a factor which is known as ‘silt factor’.

• It depends on the mean particle size of silt. It isdenoted by ‘f’. The silt factor is determined by theexpression,

Silt Factor

Coefficient of Rugosity (n) • The roughness of the canal bed affects the

velocity of flow. The roughness is causeddue to the ripples formed on the bed of thecanal. So, a coefficient was introduced byR.G Kennedy for calculating the meanvelocity of flow. This coefficient is known ascoefficient of rugosity and it is denoted by‘n’. The value of ‘n’ depends on the type ofbed materials of the canal.

Coefficient of Rugosity (n)

Mean Velocity • It is found by observations that the

velocity at a depth 0.6D represents themean velocity (V), where ‘D’ is the depth ofwater in the canal or river.

•

Mean Velocity

Critical Velocity• When the velocity of flow is such that there is no

silting or scouring action in the canal bed, thenthat velocity is known as critical velocity. It isdenoted by ‘Vo’. The value of Vo was given byKennedy according to the following expression,

• Where, D = Depth of water

Critical Velocity Ratio (C.V.R) • The ratio of mean velocity ‘V’ to the critical velocity

‘Vo’ is known as critical velocity ratio (CVR). It isdenoted by m i.e.CVR (m) = V/Vo

• When m = 1, there will be no silting or scouring.• When m > 1, scouring will occur• When m < 1, silting will occur

• So, by finding the value of m, the condition ofthe canal can be predicted whether it will havesilting or scouring

Regime Channel • When the character of the bed and bank

materials of the channel are same as thatof the transported materials and when thesilt charge and silt grade are constant,then the channel is said to be in itsregime and the channel is called regimechannel. This ideal condition is not practicallypossible.

Hydraulic Mean Depth/Ratio• The ratio of the cross-sectional area of flow

to the wetted perimeter of the channel isknown as hydraulic mean depth or radius. Itis generally denoted by R.

R = A/P

Where,• A = Cross-sectional area• P = Wetted perimeter

Full Supply Discharge• The maximum capacity of the canal for which

it is designed, is known as full supply discharge.The water level of the canal corresponding to thefull supply discharge is known as full supply level(F.S.L).

Economical Section• If a canal section is such that the earth obtained

from cutting (i.e. excavation) can be fully utilized informing the banks, then that section is known aseconomical section. Again, the discharge will bemaximum with minimum cross-section area. Here,no extra earth is required from borrow pit and noearth is in excess to form the spoil bank. Thiscondition can only arise in case of partial cutting andpartial banking. Sometimes, this condition isdesignated as balancing of cutting and banking.Here, the depth of cutting is called balancingdepth.

Economical Section

Unlined Canal Design on Non-alluvial Soil

• The non-alluvial soils are stable and nearly impervious.For the design of canal in this type of soil, thecoefficient of rugosity plays an important role, but theother factor like silt factor has no role. Here, the velocityof flow is considered very close to critical velocity. So,the mean velocity given by Chezy’s expression orManning’s expression is considered for the design of canalin this soil. The following formulae are adopted for thedesign.

Unlined Canal Design on Non-alluvial Soil

Unlined Canal Design on Non-Alluvial Soil

Problem

Problem

ProblemDesign a most economical trapezoidal section of a canal having the following data: Discharge of the canal = 20 cumecPermissible mean velocity = 0.85 m/sec. Bazin’s constant, K = 1.30 Side slope = 1.5:1 Find also the allowable bed slope of the canal SolutionLet, B=Bed Width, D= Depth of waterCross-Sectional area, A= B + 3D x D

2= (B + 1.5 D)D

Wetted Perimeter, Pw= B + 2√D2 + (1.5D)2= B +3.6 D

ProblemHydraulic mean depth, R = A = (B +1.5 D) D …..3

Pw B+3.6 DAgain, we know that for economical sectionR= D/2 …...4Therefore D/2 = (B + 1.5 D) D

B+ 3.6 D Solving it we get B= 0.6 D .…. 5Again from Q= A x VA= Q = 20 = 23.53 m2 ..….6

V 0.85 23.53= (B- 1.5 D) DOr 23.53 =( 0.6 D+ 1.5 D) Dputting the value of B in above eqn we get

D= 3.35 m

ProblemFrom eqn (5) B- 0.6 x 3.35 = 2.01 mTherefore Pw = B + 3.6 D = 2.01 + 3.6 x 3.35 =14.07 mR = -23.53 = 1.67 m

14.07By Bazin’s formula, C= 43.5From Chezy’s formula , V = C √ RS0.85 = 43.5 √ 1.67 x STherefore S = 1/ 4374 (say)So, bed width B= 2.01 m, depth of water = 3.35 m

Problem• Find the bed width and bed slope of a canal having the

following data:• Discharge of the canal = 40 cumec• Permissible mean velocity = 0.95 m/sec.• Coefficient of Rugosity, n = 0.0225• Side slope = 1:1• B/D ratio = 6.5

ProblemSolutionLet, B= bed width, D = depth of water Cross-sectionalArea, A = (B+ D) x D ……..1Wetted Perimeter, Pw= B +2√ 2 D …….2Now, A= Q = 40 = 42.11 m 2

V 0.95B/D= 6.5B= 6.5 D …….3 42.11= (6.5 D +D)DD= 2.37 mB= 6.5 x 2.37 = 15.40 mPw = 15.4 + 2 √ 2 x 2.37 = 22.10 m

ProblemHydraulic mean depth= R = A= 42.11= 1.90 m

Pw 22.20From Manning’s FormulaV= 1 x R 2/3 S ½

N0.95 = 1/ 0.0225 x (1.9) 2/3 x S ½0.95 = 44.44 x 1.534 x S ½S = 0.000194S= 1/ 5155 (say) (Bed Slope)

Unlined Canal Design on Alluvial soil by Kennedy’s Theory

• After long investigations, R.G Kennedy arrived at a theory whichstates that, the silt carried by flowing water in a channel iskept in suspension by the vertical component of eddy currentwhich is formed over the entire bed width of the channel and thesuspended silt rises up gently towards the surface.

The following assumptions are made in support of his theory:• The eddy current is developed due to the roughness of the

bed.• The quality of the suspended silt is proportional to bed width.• It is applicable to those channels which are flowing

through the bed consisting of sandy silt or same grade of silt.• It is applicable to those channels which are flowing

through the bed consisting of sandy silt or same grade of silt.

Unlined Canal Design on Alluvial soil by Kennedy’s Theory

• He established the idea of critical velocity ‘Vo’ which willmake a channel free from silting or scouring. From, longobservations, he established a relation between the criticalvelocity and the full supply depth as follows

• The values of C and n where found out as 0.546 and 0.64respectively, thus

• Again, the realized that the critical velocity was affectedby the grade of silt. So, he introduced another factor (m)which is known as critical velocity ratio (C.V.R).

Drawbacks of Kennedy’s Theory • The theory is limited to average regime channel

only.• The design of channel is based on the trial and

error method.• The value of m was fixed arbitrarily.• Silt charge and silt grade are not considered.• There is no equation for determining the bed

slope and it depends on Kutter’s equation only.• The ratio of ‘B’ to ‘D’ has no significance in his

theory.

Design Procedure

Problem

Problem

Problem

Problem

Problem

Problem

Unlined Canal Design on Alluvial soil by Lacey’s Theory

• Lacey’s theory is based on the concept of regimecondition of the channel.

• The regime condition will be satisfied if,• The channel flows uniformly in unlimited incoherent

alluvium of the same character which is transported by thechannel.

• The silt grade and silt charge remains constant.• The discharge remains constant.

Unlined Canal Design on Alluvial soil by Lacey’s Theory

• In his theory, he states that the silt carried by the flowingwater is kept in

• suspension by the vertical component of eddies. The eddies aregenerated at

• all the points on the wetted perimeter of the channel section.Again, he

• assumed the hydraulic mean radius R, as the variable factorand he

• recognized the importance of silt grade for which in introduced afactor which

• is known as silt factor ‘f’.• Thus, he deduced the velocity as;• Where, V = mean velocity in m/sec, f = silt factor,• R = hydraulic mean radius in meter

Unlined Canal Design on Alluvial soil by Lacey’s Theory

Problems

Problems

Problems

Drawbacks of Lacey’s Theory • The concept of true regime is theoretical and con

not be achieved practically.• The various equations are derived by considering

the silt factor of which is not at all constant.• The concentration of silt is not taken into account.• Silt grade and silt charge is not taken into account.• The equations are empirical and based on the available

data from a particular type of channel. So, it may notbe true for a different type of channel.

• The characteristics of regime channel may not be samefor all cases

Comparison between Kennedy’s and Lacey’s theory

Design of Lined Canal • The lined canals are not designed by the use

of Lacey’s and Kennedy’s theory, because thesection of the canal is rigid. Manning’sequation is used for designing. The designconsiderations are,

• The section should be economical (i.e. cross-sectional area should be maximum withminimum wetted perimeter).

• The velocity should be maximum so that thecross-sectional area becomes minimum.

• The capacity of lined section is not reduced bysilting.

Section of Lined Canal• The following two lined sections are generally adoptedCircular section:• The bed is circular with its center at the full supply level and

radius equal to full supply depth ‘D’. The sides aretangential to the curve. However, the side slope isgenerally taken as 1:1.

Section of Lined CanalDesign Parameters for Circular Section

Section of Lined Canal• Trapezoidal section• The horizontal bed is joined to the side slope by a

curve of radius equal to full supply depth D. The side slopeis generally kept as 1:1

Section of Lined CanalDesign Parameters for Trapezoidal Section

Problems

Problems

Exam Questions• Describe the method of design of a lined canal.• Describe the method of designing an irrigation canal based

on Lacey’s theory.• Using Kennedy’s theory, design a channel section for the

following data:• Discharge, Q=25 cumec, Kutter’s N=0.0225,• Critical velocity ratio, m=1, Side slope =1/2:1 and• Bed slope, S=1/5000

References• Irrigation Engineering

– By Prof N N Basak– Tata Mcgraw-Hill

• Irrigation Engineering & Hydraulic Structures– By Prof. Santosh Kumar Garg– Khanna Publishers

• Internet Websites• http://www.uap-bd.edu/• Lecture Notes By: Dr. M. R. Kabir• Professor and Head, Department of Civil Engineering

Department • University of Asia Pacific (UAP), Dhaka

Thanks………..GHT